BJ 24 ply panel broken during stitching

[Hugh M.]

http://www.imagestation.com/mypictures/inbox/view.html?id=4136948447&url=http%3A%2F%2Fwww.imagestation.com%2Fpictur e%2Fsraid143%2Fp8dc11925ce53bf9f27649ab91fe07306%2 Ff694d2df.jpg&caption=PICT0647 http://www.imagestation.com/mypictures/inbox/view.html?id=4136948458&url=http%3A%2F%2Fwww.imagestation.com%2Fpictur e%2Fsraid143%2Fp99187af5f8ffe6d7c878619a704b6ab2%2 Ff694d2ea.jpg&caption=PICT0645 So when I was beginning the attachment of the starboard upper side panel the forward most block that makes it lie fair point loaded the plywood and broke it in two.

Anyone have ideas on how to repair this? I don’t suppose I can simply glue the break back together. I think I’ll have to unstitch everything, lay the panel flat and set in a new scarf near the bow.

The bigger question is how to prevent this when I go to do it again? One idea I’ve heard is to bend and screw a batten around the outside near the chine line. I was also thinking of laminating a large patch of glass around the outside of the first 3 feet of the bottom panels. Of course this would have to be faired eventually.

Other ideas? Encouragements?

Thanks!
-Hugh

[Hugh M.]

Sorry, I guess the photos did not post. I am not sure how to do that.
-Hugh

[Bruce Hooke]

Was there any evidence of a defect in the plywood in the place were it broke? What kind of plywood are you using? It should take a hell of a lot of force to break a piece of plywood. If the problem seems to be the point loading, maybe you could add some additional temporary supports in the area.

As to repairing it, yes, I'd guess that you will need to take it off and scarf on a new front end, or scarf on a filler piece to fill in between the existing front end and the main part of the panel. You might be able to do this in place but it will probably be easier to unstitch it and lay it flat.

[Hugh M.]

No, there are no obvious defects upon inspection. Plywood is top-quality BS1088 Occume from Edensaw.

It broke with a lot of force - Bang! Really loud. It was also in an area where it was being bent in two directions and was actually convex on the outside of the hull.
-Hugh

[Bruce Hooke]

Take a look at the image posting instructions in this thread: http://media5.hypernet.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=005946

Pay particular attention to the instructions posted by Norm -- they are more detailed than the instructions from Bob K. that are higher up in the thread.

Having some pictures might help with figuring this out.

Is the plywood you are using the thickness that the designer specified? You might want to check with the designer if he is available to see what he recommends and to find out if other people have run into this problem.

[John Blazy]

Lets see if you pic posts now:
http://www.imagestation.com%2Fpictur e%2Fsraid143%2Fp8dc11925ce53bf9f27649ab91fe07306%2 Ff694d2df.jpg

Sorry - don't know what the problem is - tried to paste only part of the code, dropping everything after ".jpg"

All I do is pull up the picture in imagestation, right click on it, scroll down to click on 'properties', then copy the code, making sure not to copy any code after ".jpg".

Anyway, if the ply breaks then its bad design - way too much stress on that part of the ply, even if it was successful. Sounds like you're "torturing" the ply to create "flare", which is OK, if its not too much. How thick is the ply, how many plies (layers) and what grain direction is the outer skins (assuming along the length)?

If you scarf in a new piece, its almost guaranteed to break again at the scarf if the stress was enough to snap raw ply. You might want to plane off some of the inner ply layers to help it bend easier, then fill the inside area later with glass & epoxy.

If you try scarfing in new, remember that all scarf joints should not run perpendicular to the length, but rather at an angle (like this pic below) so the bend stresses aren't concentrated at the joint:
http://www.imagestation.com/picture/sraid46/p2fe5b45681b2cc1916058e4ad9b9f598/fccc9dff.jpg
Also make sure to prewet the epoxy in the scarf before clamping so you don't starve the joint, and it helps alot to apply glass cloth to the outside of the scarf as insurance.

Without seeing the pic I could be way off though.

[Paul Scheuer]

http://www.imagestation.com/picture/sraid143/p8dc11925ce53bf9f27649ab91fe07306/f694d2df.jpg

John:
Sometimes the /'s get converted to %2F's by some auto edit scheme.
Hugh:
If you pick the URL from "image properties" (rather than the toolbar) you get the correct code, without the extra stuff.

[ 10-19-2004, 11:23 PM: Message edited by: Paul Scheuer ]

[Gary Lee]

Have you spoken to the designer directly?

[capt jake]

Looks like it broke on the side you weren't stitching, correct? I had thought is snapped on the stitched side. Interesting........

[Tom Lathrop]

Since I designed the Bluejacket 24 I guess it is time that I got into this problem after just talking with Hugh.

The break was caused by stress concentration caused by the assembly sequence. The fact that the bottom panels were opened and took their design shape indicates that there was no undue stress up to that point. When the port topside panel was stitched in place and the fairing blocks screwed in, the hull undoubtedly warped a bit and put more stress on the starboard side bottom panel. Hugh then attempted to fit the starboard topside panel in place and screw the fairing blocks down BEFORE tightening the wire ties. this combination put a stress concentration on the already stressed bottom panel just in front of the block as seen in the photo and the panel failed.

The fact that Hugh is using 9 ply plywood may also have contributed since it is said to be stiffer than 7 or 5 ply.

The proper sequence to install a panel that needs bend and/or twist is to start at the end with the most severe bend or twist. This is almost always at the bow. All wire ties should be put in from this point and tightened snug. This will help prevent stress concentration by allowing some transfer of stress to adjacent areas as the panel is gradually bent into shape. The fairing blocks should not be put in until all the panels are in place and wires tightened. The purpose of these blocks is to force the panels to make a fair intersection at the chine near the bow, which they may not like to do.

In severe cases, installing a batten an inch or two below the chine and on the outside will be even more effective in preventing stress concentration. Another help is to squegee on a thin coat of epoxy on the outside (tension) side of the curvature which will delay local fiber failure by spreading the stress. Sometimes water and/or heat can help but on thick plywood, it may not work too well. There are limits to which plywood can be bent without fracture but using these techniques will enable you to reach that limit.

Some more stuff to be added to the plans I guess since not everone is familiar with these problems or solutions and they are not in any of the books I know of. At least that is fairly easy for me to do since I make my plans in a loose leaf format that is not too much work to add to or modify.

[ 10-19-2004, 10:15 PM: Message edited by: Tom Lathrop ]

[John Blazy]

Wow - that must have been loud. Did you spec 9 ply Tom? Seems way overkill. It also appears (hard to judge from a photo) to have broke at the apex of a rather abrupt, tight radius. Gotta smooth that bump out of the curve a little and it should work fine next try (with new ply).

[Tom Lathrop]

Originally posted by John Blazy:
Wow - that must have been loud. Did you spec 9 ply Tom? Seems way overkill. It also appears (hard to judge from a photo) to have broke at the apex of a rather abrupt, tight radius. Gotta smooth that bump out of the curve a little and it should work fine next try (with new ply).No, 9 ply is not specified but would not be overkill.

There is no abrupt change in curvature in the design of the panel.

I'm sure that Hugh will have no trouble making this right after he scarfs in a new section.

[John Blazy]

Great advice Tom, on starting the stitching from the bow rather than aft. Hope all works out.

[Hugh M.]

Thanks to all who contributed - especially to Tom who called me long distance to personally talk me through it.

As to the number of plys, I was not given a choice on that. I simply ordered 1/2 inch Occume and 9 ply is what was delivered.

As to whether or not 9 is too stiff - I don't think it is. The break occured right at a temporary block and was due to the force imparted by the upper panel bend transfering down into the lower panel through the fairing block with little other support.

Live and learn.
-Hugh

[Paul Scheuer]

Here's the other view.
http://www.imagestation.com/picture/sraid143/p99187af5f8ffe6d7c878619a704b6ab2/f694d2ea.jpg

[Henning 4148]

Some time ago in another magazin it was suggested to pour boiling water over the ply in areas that have a lot of curvature to make it softer. I have used that trick not on a boat but on a wooden cover and it seems to work, but that was 5 mm 3 layers ply.

[capt jake]

Hugh, how is the repair coming along? George, anything to share here on your build??

[George Roberts]

Hugh M. ---

Tom commented that you used 9 ply material.

Assuming the ply materials are identical and Fb is the controlling factor ...

The layers in 9 ply material are only 7/9ths the thickness of of 7 ply material. A consequence of this is that 9 ply material will only support 7/9ths of the load in bending that 7 ply will support.

This places your "better" material a distant second where bending is important.

[capt jake]

Assuming the ply materials are identical and Fb is the controlling factor ...

The layers in 9 ply material are only 7/9ths the thickness of of 7 ply material. A consequence of this is that 9 ply material will only support 7/9ths of the load in bending that 7 ply will support.

This places your "better" material a distant second where bending is important. OK, a couple of questions;

What is Fb?

In my product research, the 7 ply Okume is rated 6566 while the 9 ply is rated 1088. So you are suggesting going with 6566 because if the number of plys, correct? At least on the 'bent' panels?

The above number of plys only applies to the 1/2" material. The 3/8" had a lesser number of plies, I think it was 5 and 7.

[Hugh M.]

Progress has been slow - had to get over my despondancy. OK, now I am over it! Just scarfed in new ply on Sunday. I will be cutting out and re-stitching soon.
-Hugh

[George Roberts]

capt jake ---

Fb (usually written lower case "f" subscript "b") is the maximum fiber stress allowed. It is also the tension at the top (compression at the bottom) of the panel due to bending.

The standards you quote have to do with the construction of the plywood and may or may not affect the allowable Fb. (The APA web site has information on computing bending properties of construction and marine plywood, but the properties are not fine enough to answer most questions.)

The rest of the question I will pass on. You should use what the designer recommends and complain to him if it does not work out.

[NormMessinger]

Originally posted by Hugh M.:
... - had to get over my despondancy. ...
-HughBeen there done that. When Lynn and I tried to make a canoe to the Sweet Dreams plans I mistakenly used plywood that was too thick. Man! When that stuff let go it dang near caused heart failure. In our case there was no way to recover. Scraped the wood and started over.

Joy to you though. Next week no one will know the difference.

[Tom Lathrop]

Hugh,

Everyone who bends wood, and especially plywood, is going to experience the trauma of an explosion occasionally if they are working anywhere near the limits of the fiber stress. While knowing the fiber stress factor may be interesting, it will not do you much good with an individual piece of plywood since each piece has its own limits. This is particularly true with plywood of different constructions. Some plywoods have plys of equal thickness and others have thinner surface plys. As George said, the tension stress where the problem usually arises is only in the layer on the outside of the bend.

Norm,

I see that you are a fellow traveler in this situation. The explosive bang does make you flinch doesn't it? redface.gif

Using the techniques I gave in my above post will allow bending even past the normal limits as stress concentrations are not allowed to develop. In Hugh's case, he actually introduced a stress not anticipated by the plans and instructions. It is important to remember that Hugh had no problem in bending the offending panels in the first place. The problem arose when a point stress was introduced by the way the topside panel was mounted. The instructions give the proper sequence but there is apparently not enough emphasis on the consequences of not following it. What was very clear to me was less clear to Hugh. Anyone who writes instructions will have this happen. As I said, I will caution other builders about this possibility.

Crap happens :(